I think can address one potential problem of adding an outrigger/poa to a mono to replace the keel
First, keels do not
keep your boat upright. It's the opposite, your keel
is constantly helping the wind
your boat. What the keel does do it keep your boat from sliding sideways. The only things keeping monos upright are the hull
shape and ballast.
*I'm talking about the keel as a conceptually different thing from the ballast, I realize that structurally they are combined.
The job of the poa/outrigger would be the same as the hull
shape/ballast, and it would do a very poor job of keeping the boat from sliding sideways. This is why the hull forms on the main hull for these types of boats are always slender. Basically I think the poa/outrigger would be a poor replacement for a keel (without drastically changing the main hull shape) because it servers a different function so you'd still need a centerboard
My general thought on why we don't see these boats much internationally and on the cruising scene is because they aren't well balanced...literally. There are potential benefits as long as the additional hull is leeward because compared to the symmetrical craft, the arm of the righting moment is relatively longer and relies on buoyancy. When the additional hull is on the windward side, you're relying on the sheer weight of it to keep you from capsizing, this is much the same as a monohull
. Given the choice between making something artificially heavy, or simply adding another light buoyant force to the other side, it seems to be more efficient to do the latter rather than the former.